1. Field of the Invention
The present invention relates to receiver synchronization, and more particularly to code tracking in a CDMA system having, but not exclusively, application to the field of mobile telephones.
2. Description of the Prior Art
In a radio communication system the symbol clocks of the transmitter (Base Station, BS) and a receiver (Mobile Station, MS) are unsynchronized. A synchronization method for the receiver is thus needed to synchronize the symbol timing i.e. the symbol clock in a receiver to the received symbol sequence. In CDMA system the symbol synchronization is more often called chip synchronization since the synchronization between the receiver chip clock and the received chip sequence is determined. Moreover, in a CDMA system a method is needed to keep the receiver spreading sequence aligned to that of the transmitter (BS), so the method is also referred to as code tracking.
This synchronization, in general, is achieved by extracting a suitable control signal from the received signal, and using a phase locked loop to keep the error between the extracted control signal and a locally generated copy of the control signal as small as possible. One such known technique is based on Delay-Locked Loop (DLL) where the combined impulse response of the transmitter filter, communication channel and receiver filter is computed in the receiver based on the received signal. The DLL then tries to minimize the difference between the delay of the channel and the reference delay of the locally generated impulse response.
One known DLL tracking method is called as Early-Late DLL method where one sample of the impulse response is calculated half of a chip earlier and another sample is calculated one half of a chip later than the desired sampling point.
Another method, called as Early-Ontime DLL, is known from U.S. Pat. No. 5,590,160 where one sample of the impulse response is calculated half of a chip earlier than the desired sampling point and another sample is calculated at the desired sampling point. The DLL then uses the sample values in phase locked loop to control the synchronization timing. In U.S. Pat. No. 5,590,160 a DLL operation is disclosed where the ratio of these samples is compared to a reference ratio, and the result is used as an error signal for the phase locked loop.
One problem in a wireless communication relating to synchronization is the multipath propagation. In that situation the received signal is a superposition of several versions of the transmitted signal, delayed by various delays with the signal versions having random phases and amplitudes. The impulse response is consequently also a superposition of several versions of the combined transmitter and receiver impulse responses, which means that the original impulse response is distorted by this superposition. This distortion is particularly difficult if the delay between two or more impulse responses is between 0.2–2 symbols (or chips in CDMA). This condition is commonly known as a “fat finger” case. If the delay difference is smaller than 0.2 symbols (chips), the impulse response is not that distorted but this case can cause a situation called flat fading where different signal versions cancel each other out. On the other hand, if the delay is more than 2 symbols, the impulse responses do not effect each other much and the combined impulse response may be viewed as two separate impulse responses.
In U.S. Pat. No. 5,590,160 the Early-Ontime DLL is used in a spread spectrum receiver where the receiver consists of a several receivers, known as fingers. Each finger is used to demodulate its own version of the delayed signal, and the output from the individual fingers is coherently combined. This type of receiver is commonly known as a RAKE receiver. In a RAKE receiver the DLL is used to lock each separate finger to a distinguishable signal copy. However, when the above mentioned fat finger case happens neither Early-Ontime nor Early-Late DLLs can provide optimal operation since they do not allow all of the energy in the fat finger to be demodulated. The problem is that DLLs in each finger are locking the fingers to the same position, thus providing no gain. In other words, although these different DLLs (Early-Ontime and Early-Late DLLs) are locking on to different places of the fat finger, the demodulated energy is however roughly equal in both cases.